With the recent wide spread use of portable electronic devices such as mobile phones and laptop computers, the development of small and light-weight nonaqueous electrolyte secondary batteries having high energy density has been strongly demanded. Moreover, the development of secondary batteries with high output energy serving as the batteries for electric vehicles typified by hybrid vehicles has been strongly demanded. One of the secondary batteries that satisfy these demands is a lithium ion secondary battery. A nonaqueous electrolyte secondary battery typified by a lithium ion secondary battery includes a negative electrode, a positive electrode, an electrolyte solution, and the like, and contains a material capable of intercalation and deintercalation of lithium as the active material of the negative electrode and the positive electrode.
The positive electrode active material for a nonaqueous electrolyte secondary battery has been extensively researched and developed recently. Above all, the practical application of the nonaqueous electrolyte secondary battery whose positive electrode material contains a lithium-containing composite oxide with a layered structure or a spinel structure has been advanced as the battery with the high energy density because a voltage as high as 4 V can be obtained.
A number of developments have been conducted on the battery containing lithium cobalt composite oxide (LiCoO2), which can be synthesized relatively easily, as the positive electrode material of the lithium-containing composite oxide in order to achieve an excellent initial capacity characteristic or cycle characteristics, and various results have already been obtained. However, the lithium cobalt composite oxide is formed using a cobalt compound for its raw material, and since the cobalt compound is rare and expensive, the cost for the active material and moreover the batteries will increase. In view of this, the replacement for the active material has been expected.
Thus, lithium nickel composite hydroxide (LiNiO2) containing nickel, which is less expensive than cobalt, has attracted attention and a higher capacity can be expected from this material. The lithium nickel composite hydroxide is excellent in terms of cost, and moreover has a lower electrochemical potential than the lithium cobalt composite oxide; therefore, the decomposition of the lithium nickel composite hydroxide due to the oxidation of the electrolyte solution is less problematic. In addition, the lithium nickel composite hydroxide can be expected to have a higher capacity and exhibits a battery voltage as high as that of the lithium cobalt composite oxide; therefore, the development of the lithium nickel composite hydroxide has been extensively carried out. The nonaqueous electrolyte secondary battery employing the lithium nickel composite hydroxide containing only nickel as a metal other than lithium has some drawbacks: the cycle characteristic is inferior to that of the nonaqueous electrolyte secondary battery containing the lithium cobalt composite oxide; and the battery performance easily deteriorates if the nonaqueous electrolyte secondary battery employing the lithium nickel composite hydroxide is used or preserved under a high-temperature environment.
In order to overcome such drawbacks, for example, Patent Literature 1 has disclosed the positive electrode active material for a nonaqueous electrolyte secondary battery, which is represented by LiNi1-x-yCoxZnyO2 (0<x≤0.20, 0<y≤0.13), containing hexagonal lithium-containing composite oxide with a layered structure, and having 3% or less of the site occupation ratio of metal ions other than lithium in 3a-site.
Patent Literature 2 has provided the positive electrode active material for a lithium secondary battery including a plurality of primary particles containing lithium composite oxide with a layered rock salt structure containing at least lithium, nickel, and an element selected from cobalt and zinc. In the provided positive electrode active material for a lithium secondary battery, at least one of the positive electrode active material for a lithium secondary battery and the primary particles has higher cobalt concentration in a part near the surface than the inside, and the cobalt element may be partly or entirely replaced by the zinc element.